A Stringent Upper Limit of the PH3 Abundance at the Cloud Top of Venus

International audienceFollowing the announcement of the detection of phosphine (PH 3 ) in the cloud deck of Venus at millimeter wavelengths, we searched for other possible signatures of this molecule in the infrared range. Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO 2 and H 2 O at the cloud top. We identified a spectral interval recorded in March 2015 around 950 cm ? 1 where a PH 3 transition is present. From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3- ? upper limit of 5 ppbv for the PH 3 mixing ratio, which is assumed to be constant throughout the atmosphere. This limit is four times lower than the disk- integrated mixing ratio derived at millimeter wavelengths. Our result places a strong constraint on the maximum PH 3 abundance at the cloud top and in the lower mesosphere of Venus